Method of treating interlayer material

ABSTRACT

This invention relates to making continuous ribbons of clear, flexible interlayer material into a maximum number of interlayers having a coating of graded intensity along a longitudinal edge portion thereof for use in curved laminated safety glass windshields. The method avoids rubbing or differentially stretching the flexible interlayer material during processing, which includes cutting a continuous ribbon of clear, flexible interlayer material into successive interfitting flexible sheets of trapezoidal shape, applying a shade band, preferably by electrostatic spraying, to a predetermined portion of each sheet and laminating the sheet so treated to one or more rigid transparent sheets of glass or a recognized plastic substitute for glass. 
     When the flexible sheets so treated are laminated to one or more relatively rigid transparent bent sheets of glass or rigid transparent plastic, the resulting laminates are safer than those produced by the prior art method that requires differentially stretching the partially shaded interlayer and have better optical properties than prior art shaded laminates comprising interlayers produced by previous prior art techniques that incorporated rubbing the sheet while applying a coating.

This is a division of the U.S. Patent Application Ser. No. 887,563,filed Mar. 16, 1978, now U.S. Pat. No. 4,244,997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to curved laminated transparencies comprising aflexible interlayer having a shade band of uniform tone but a pattern ofgraded intensity laminated to one or more rigid, transparent sheets ofglass or rigid transparent plastic, and to preparing a thermoplasticinterlayer for a curved laminated glass assembly having a colored,light-absorbing band in a marginal area of its thermoplastic interlayer.The present invention more especially relates to a method for thepreparation of individual interlayers from a continuous ribbon of clearflexible interlayer material and to the use of such interlayers inlaminated windshields having more acceptable optical properties and/orimproved resistance to impact than those containing interlayers producedby other methods developed in the prior art.

Windshields of automobiles utilize laminated glass assemblies comprisingtwo sheets of glass and a thermoplastic interlayer such as a sheet ofplasticized polyvinyl butyral resin or polyurethane. In the currentmodels of automobiles these glass assemblies are bent along thelongitudinal axis. In the manufacture of these curved laminated glassassemblies, a thermoplastic interlayer is placed between two matchedcurved glass sheets and the resulting sandwich is subjected to alaminating process to form a transparent laminated glass assembly. Thethermoplastic interlayer is provided with a marginal rectilinear orstraight band of a light-absorbing material in order to reduce glarefrom the sun when the laminated glass assembly is mounted as awindshield in an automobile. The marginal band is preferably graduatedin concentration of light-absorbing material with the greatestconcentration of light-absorbing dye being present nearest the upperperipheral portion of the thermoplastic interlayer and the dyeconcentration in the band diminishes gradually toward the lower edge ofthe band until the amount at the lower edge is almost imperceptible.When such a graduated dyed band in the plastic interlayer is laminatedbetween a pair of curved glass sheets and the resulting laminatedwindshield is mounted in a tilting or non-vertical fashion, the cut-offpoint between the dyed band and the undyed portion appears to be curvedto a person inside the car. In order to provide an apparent horizontalcut-off line between the dyed band and the non-dyed portion, it isnecessary to distort the flat thermoplastic interlayer sheet to curvethe boundary between the dyed and the undyed portion in such a manner asto compensate for the curvature that results from differentialdistortion of the interlayer sheet.

Flat thermoplastic interlayer sheets having a curved colored bandsuitable for use as interlayers in curved laminated glass assemblieshave been prepared from a continuous thermoplastic sheet having astraight colored band by differential stretching of the sheet until thecolored band is curved and then heating the sheet while in the stretchedcondition in order to relieve stresses. The sheets are then cooled toroom temperature while still in a stretched condition. Such a method islimited with respect to the amount of stretch that can be used as wellas with respect to the degree and type of curvature obtainable.

Thermoplastic sheets stretched differentially by the method describedhave a non-uniform thickness even when cut from a continuous ribbon ofinterlayer material of uniform thickness. Furthermore, the heating ofthe differentially stretched sheet does not eliminate the stressesentirely.

The combination of the residual differential stresses and the differencein thickness resulting from the differential stretching of thethermoplastic sheets (which usually vary about 5 mils (0.13 mm) ininterlayer sheets of 30 mils (0.762 mm) nominal thickness) makes itdifficult to obtain a uniform bonding between the thermoplasticinterlayer sheets and a glass sheet or a pair of glass sheets to producea laminated safety glass windshield. Laminating assemblies havinginterlayers of non-uniform thickness in an oil autoclave may result insome oil penetration at the interface between the thinner portion of theinterlayer and the glass or other rigid transparent sheet. Furthermore,differential stretching causes certain uneveness in the gradation ortexture of the band of shading that is applied to the interlayermaterial. The art of shaded laminated automobile windshields requiredsome improvement to provide tinted windshields that were not susceptibleto delamination and had better optical properties than those availablefrom the prior art.

2. Description of the Prior Art

U.S. Pat. No. 3,455,197 to Richardson and U.S. Pat. No. 3,467,332 toBachman disclose different types of apparatus for cutting a continuoussheet or ribbon of clear plastic interlayer material into interfittingtrapezoidal shapes. Such apparatus are designed for ultimate use of theclear plastic in windshields that do not have dyed portions.

U.S. Pat. No. 3,113,034 to Fix discloses an axially displaceableprinting roller whose axial reciprocating movement is correlated withthe movement of a continuous ribbon of plastic interlayer material toprovide a graded pattern of dye that is repeated along the length of thecontinuous ribbon. Such ribbon must necessarily be cut with triangularend portions that do not interfit. Furthermore, the axial reciprocationof the printing roller forms rub marks that impair the opticalproperties of the printed portion of the interlayer.

U.S. Pat. No. 3,593,405 to Beckham discloses a typical prior arttechnique involving the application of a shade band of graded intensityonto a rectangular sheet of plastic interlayer material. The interlayermaterial containing the shaded portion of graded intensity isdifferentially stretched to provide a curved cut-off line so as toappear substantially horizontal when laminated between curved glasssheets and installed in a tilted position in a frame of an automobile.

U.S. Pat. No. 2,933,759 to Startzell discloses an umbrella-typeapparatus for simultaneously differentially stretching the opposite sideedges of a plurality of layers of a continuous ribbon of plasticinterlayer material so that the plurality of layers may besimultaneously differentially stretched when the umbrella-type ofplastic stretching apparatus is opened. Stretch marks usually remain inthe interlayer sheeting. When the shape requires severe differentialdistortion, these can extend into the shaded region.

U.S. Pat. No. 3,696,186 to Stark et al discloses apparatus fordifferentially stretching a continuous ribbon of dyed interlayermaterial by passing the latter while heated to deformation temperatureover a portion of a conical roll and then rapidly chilling thedifferentially stretched plastic interlayer material. Non-uniformstretching results in non-uniform thickness.

The teachings in the patents enumerated and described previously fail toprovide a method of treating clear interlayer material so that a maximumnumber of interfitting trapezoidal shapes can be provided with a gradedshade band along a specific longitudinal edge portion thereof forlamination with curved glass to result in a laminated glass-plasticwindshield having a plastic layer of substantially uniform thickness, ofsubstantially uniform tension of a low magnitude merely sufficient toavoid wrinkling and of substantial uniformity of adhesion to the glassthroughout the entire extent of the laminated windshield.

SUMMARY OF THE INVENTION

According to the present invention, a continuous ribbon of plasticinterlayer material of substantially uniform overall thickness is cutinto successive lengths of interfitting trapezoidal shapes. The ribbonis preferably embossed to avoid entrapping air in the resultinglaminate. The individual sheets so formed are oriented into apredetermined orientation. This may be accomplished after stackingalternate trapezoidal sheets in separate layers and orienting each stackto facilitate removing each sheet from its associated stack in itsdesired predetermined orientation.

Each sheet has a graded coating applied to a portion thereof while sooriented. The sheet is supported in unwrinkled condition with a minimumof stress applied thereto during the application of the coating. Eachsheet of trapezoidal shape of said plastic interlayer material issupported in such a manner as to avoid wrinkling and differentialstretching during the step of applying the coating. A coating of gradedintensity is applied to a preselected longitudinal portion only of eachtrapezoidal sheet so that the resulting coated interlayer sheet has asubstantially uniform thickness (except for an embossment pattern) andhas a substantially uniform stress of small magnitude sufficient toavoid wrinkling throughout its entire extent. The non-uniformapplication of the graded coating does not appreciably affect theuniform thickness and substantially uniform stress of the trapezoidalsheets. The uniform thickness improves the ability of the interlayer tobe adhered or bonded uniformly to a curved glass sheet with which itforms a laminated safety glass windshield with minimum chance for oilpenetration at the portion of the interface between a thinner portion ofa differentially stretched interlayer and the adjacent glass sheetduring fabrication of the laminated windshield and minimizes the chancefor delamination due to local points of high stress which serve asorigins of delamination in the laminated windshield. These points ofhigh stress may be the result of differential stretching during thedistortion of the plastic interlayer in order to convert a straight linecut-off between the shaded and clear portions of plastic or may be dueto the inability of the glass to laminate successfully against thethinner portions of the plastic that result from the differentialstretching of the plastic.

The present invention will be better understood in the light of adescription of a specific embodiment thereof that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which form part of a description of an illustrativeembodiment of the present invention and wherein like reference numbersrefer to like structural elements,

FIG. 1 is a schematic plan view of a roll of continuous plastic ribbonof interlayer material showing how the clear plastic ribbon is cut intointerfitting lengths or sheets of interfitting trapezoidal shapes;

FIG. 2 is a plan view of a pair of sheets of trapezoidal shape showinghow all the sheets of a stack are oriented preparatory to having acoating of graded intensity applied along a longitudinal area thereof;

FIG. 3 shows a sheet of flexible interlayer material with the coating ofgraded intensity applied along a longitudinal extending portion thereof;

FIG. 4 is a frontal view of a laminated glass windshield with theinterlayer bonded thereto;

FIG. 5 is a cross-sectional view taken through a laminated window ofFIG. 4 showing how the shade band of graded intensity appears to have ahorizontal cut-off line when installed in a vehicle between a pair ofbent glass sheets of matching curvature.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the present invention, a continuous roll of flexibleplastic interlayer material, such as polyurethane or plasticizedpolyvinyl butyral, is unrolled from a roll 10 to form a continuousribbon 12. The ribbon 12 is cut into interfitting alternate sheets oftrapezoidal shape 14 and 16 while the ribbon 12 is smoothly supported inan unwrinkled condition. A typical apparatus for treating a continuousribbon of clear plastic is disclosed in U.S. Pat. No. 3,455,197 toRichardson. It is also feasible to wind the continuous ribbon about aspecial mandrel such as disclosed in U.S. Pat. No. 3,467,322 to Bachman.In the latter apparatus, a plurality of layers are wound in unwrinkledcondition around the periphery of the mandrel and each circumference ofthe ribbon is cut at approximately two diametrically opposite locationsto form a pair of interfitting trapezoidal sheets from each layer.

After the sheets 14 and 16 of trapezoidal shape are cut and separatedfrom the continuous ribbon 12, the alternate sheets 14 are stacked inone stack 15 in one common orientation and the alternate sheets 16 arestacked in another stack 17 oriented in another common orientation. Theneach sheet 16 in turn is removed from stack 17 and applied in a desiredorientation while at a uniformly but lightly stressed condition merelysufficient to avoid wrinkling on a supporting table, which may be avacuum supporting table (not shown). Each sheet 14 in the stack 15 isoriented to the same orientation as stack 17 before each sheet 14 istreated similarly after the sheets 16 are depleted from stack 17. FIG. 2shows how both stacks are oriented for sheet removal.

Each plastic sheet 14 or 16 of trapezoidal shape is brought to a dyeapplying station where a band of dye is applied to an area 20 that isexposed to a spray composition while the remaining areas 22 and 24 ofthe trapezoidally shaped sheets are shielded from contact with the spraycomposition.

The dyed area 20 of the substrate has a convexly curved bottom margin 26of low intensity and an upper concavely curved margin of high intensity.The upper concavely shaped high intensity dyed boundary 28 conforms tothe shape of the upper edge of one or more glass sheets to be laminatedto the dyed interlayer material to form a bent laminated windshieldhaving a graduated shade band that is graduated in intensity transverseto the longitudinal dimension of the windshield. The convexly shapedlower border 26 of the shade band area 20 is barely noticeable as theintensity of the dye applied decreases in a direction transverse to thelongitudinal dimension of each sheet with a maximum intensity at itsconcave upper border and a minimum intensity along its convex lowerborder.

The curvature of the lower border 26 is so chosen that when thewindshield comprising inner and outer glass sheets 30 and 32,respectively, is mounted in tilted relation in a windshield frame, asshown in FIG. 5, the convexly curved lower border 26 between the shadedarea and the unshaded area 22 that is curved from the application of thedye appears as a horizontal line.

One of the important features of this invention is that during thehandling of the plastic continuous ribbon or the individual sheets oftrapezoidal shape that are cut from the plastic that it is held insmooth, slightly stretched relationship where the stretching is uniformthroughout the entire extent of each plastic sheet that is cut from thecontinuous ribbon. It is also essential that the ribbon itself bemaintained in a smooth, unwrinkled, slightly but uniformly stretchedcondition during the cutting operation. In this manner, the uniformityof thickness of the ribbon as received from the manufacturer of theribbon (which approximates 30 mils (0.76 mm)±5 mils (0.13 mm) withembossments approximately 1 mil (0.025 mm) thick) is not distorted. Theadhesion throughout the entire extent of the trapezoidal sheets ofinterlayer material to the glass is substantially uniform throughout theentire extent by virtue of the relative uniformity of thickness of theinterlayer material of the laminated assembly.

During coating, the properly oriented sheet of flexible interlayermaterial, such as polyurethane or plasticized polyvinyl butyral, issupported directly on an apertured vacuum platen having a flat uppersurface and vacuum is applied to hold the flexible sheet in a fixedposition in an unwrinkled condition on the platen. The dye is appliedobliquely and the nozzle of the spray gun may have its orifice modifiedto ensure a desired pattern of non-uniform intensity transverse to thelongitudinal dimension of the substrate. Preferably, an electrostaticspray gun is used to provide finely divided particles of dyecomposition.

The coating station is usually enclosed and provided with a conventionalexhaust hood (not shown) to remove any portion of the spray that doesnot adhere to the substrate. If desired, heat lamps may be provided toirradiate the coated portion of the substrate to help volatilize thevolatile component of the dye composition applied.

It is noted that the boundary between the coated portion and theremainder of the flexible plastic sheet that is uncoated is curved. Thiscurvature may be obtained by curving the edge of the masking means thatis interposed between the spray gun and the portion of the substratedesired to be free of coating. If desired, a curved cam may be used toguide the reciprocating movement of one or more spray guns to providethe curved cut-off line between the coated and uncoated portions of theflexible interlayer sheet.

Suitable dye solutions comprise a mixture of organic dye componentsblended to yield a desirable color. A preferred dye mixture is a blendof blue, yellow and red-violet dye components. A preferred blue dyecomponent comprises an anthraquinone derivative, such as1,4-diethylamino-anthraquinone. A preferred yellow dye component is amonoazo compound with a molecular formula of C₁₇ H₁₆ O₂ N₄. A preferredred-violet dye component appears by infrared analysis to be ananthraquinone derivative comprising an amine functionality. However,positive identification of the latter component is not obtainable. Apreferred blend of the preferred dye component, 37 parts by weight ofSolvaperm Red-Violet R, available from American Hoescht Corporation, 37parts by weight of Calco Oil Blue N and 26 parts by weight of CalcoYellow G Concentrate available from American Cyanamid Corporation,yields a relatively colorfast blue-green shade band. This dye mixture orblend of dye components is dissolved in a concentration preferably about1 to 2% by weight of dye components in a solvent system, which ispreferably a combination of tetrahydrofuran and N-loweralkyl-pyrrolidone, preferably containing about 75 to 85% by volume oftetrahydrofuran and about 25 to 15% by volume of N-methylpyrrolidone.This solvent system satisfies the requirement of high dye solubility,preferably greater than 2%, and proper volatility to assure opticaluniformity in the shade band. Non-uniformity in the shade band is causedby both too little volatility, which results in a mottled texture, andtoo high volatility, which results in undissolved dye particles beingphysically bonded to the surface of the substrate. The solvent systemsuggested is also an acceptable solvent for antioxidants and ultravioletstabilizers which are preferably added to the dye component.

A typical dye composition comprising 38.5% by weight SolvapermRed-Violet R, 38.5% Calco Oil Blue N and 23.0% Calco Oil Yellow GConcentrate (both Calco dyes available from American Cyanamid) isdissolved in a hundred milliliters of solvent for each 1.5 grams of theabove dye composition. The solvent consists of 85% by volume oftetrahydrofuran and 15% of N-methyl-pyrrolidone. The solution thusformed is electrostatically sprayed on an unmasked portion of a sheet ofpolyurethane or plasticized polyvinyl butyral at an oblique angle toyield an optically uniform blue-green shade band in the lengthwisedirection of the shade band and a graded intensity of shade band from amaximum intensity at one edge of the shade band to a relatively lowintensity adjacent a convexly curved cut-off line with an uncoatedportion beneath a mask during the coating.

Another suitable dye composition comprises 1.2 grams of the previous dyecomposition in 100 milliliters of a solvent system consisting of 80% byvolume of tetrahydrofuran and 20% by volume of N-methyl-pyrrolidone andfurther comprising in addition to the solvent, 0.12 grams of anantioxidant, Irganox 1035 and 2.4 grams of an ultraviolet lightstabilizer, Tinuvin 770, both available from Ciba-Geigy Corporation. Thelatter solution is also electrostatically sprayed to yield a relativelycolorfast shade band having the desired gradation in one direction anduniform intensity in the other direction.

After the sheet of interlayer material is provided with the elongatedshade band area, while avoiding wrinkling and differentially stretchingsaid interlayer material during its handling it is removed from thecoating area, maintained in a room of controlled temperature andhumidity, preferably about 68° F. (20° C.) and 20% relative humidity andassembled as an interlayer between a pair of glass sheets of matchingcurvature with the boundary of the shaded area having relatively highintensity aligned with an edge of the curved glass sheet and the cut-offline of relatively less intensity aligned with marks previously made inthe bent glass sheet. Excess plastic is trimmed and cut away from themargin of the assembly. The assembly of dyed interlayer material and twoglass sheets is then prepressed by passing the assembly between a pairof prepressing rolls of the type depicted in U.S. Pat. No. 3,351,001 toAchkio, while at a temperature of 150° F.-225° F. (65° C.-107° C.) toremove air from between the interfaces of the assembly and to smooth theembossed pattern and to seal at least the peripheral edge portion of theinterfaces between the components of the assembly. The rolled,prepressed assembly is then laminated in an oil autoclave under standardconditions, which involve temperatures of 225° F. to 300° F. (107° C. to149° C.) and a simultaneous pressure of 175 psi to 225 psi (12.3 to 15.8Kg/cm²) for 15 to 60 minutes depending on the other parameters and theseverity of shape of the assembly.

The tinted laminated windshields that result from the aforesaidoperation have superior optical properties and superior resistance tooil penetration during fabrication and to delamination during subsequentuse than windshields containing partially dyed interlayers that aredifferentially stretched prior to their assembly. The assembly producedaccording to this invention need not be edge rolled according to themethod taught in U.S. Pat. No. 2,999,779 to Morris, which thickens themargin of the interlayer to improve the resistance to oil penetration atthe interfaces during exposure to a pressurized oil autoclave. When theglass-plastic assembly containing an unwrinkled interlayer preparedaccording to the present invention is edge rolled, the marginal portionof the interlayer so treated develops a frame of different colorintensity than the remainder of the shade band. The uniformity ofthickness of the interlayer sprayed with a dye composition along an edgeportion while in an unwrinkled condition avoids the need to thicken theperipheral portion preferentially.

If desired, the partially dyed sheet of flexible interlayer material maybe laminated to a single glass sheet utilizing a parting material and asecond glass sheet which is separated from the interlayer afterlamination according to the technique disclosed in U.S. Pat. No.3,808,077 to Rieser and Chabal. The description of the method of forminga bilayer windshield disclosed in this latter patent is incorporatedherein by reference for the details of such an operation. Fabricating abent bilayer windshield having an outer glass sheet with an exposedconvex major surface is similar to fabricating a bent trilayerwindshield with two outer bent glass plies except for interposing alayer of parting material between the interlayer sheet and the innerconvex major surface of the inner glass sheet having an exposed concavemajor surface. The parting material facilitates removal of the innerglass sheet after lamination.

The resulting laminated windshields produced according to the presentinvention are characterized by better adhesion between the shadedplastic and the glass. It is believed that this is the result of themore uniform tensioning of the interlayer material with a small tensionsufficient only to prevent wrinkling and of substantially uniform stressthroughout the entire interlayer. The interlayer sheets, which have asubstantially uniform thickness in the roll from which they are cut, arenot distorted so that different portions thereof have differentthicknesses, which makes the problem of adhering to the other elementsof the laminated windshield exceedingly difficult. In other words,having a plastic interlayer of more nearly uniform thickness throughoutprovides an improved adhesion throughout the entire extent of thelaminated windshield that results.

While the present invention is not so limited, typical thicknesses forthe outer glass sheet of a bilayer windshield and for the inner andouter glass sheets of a trilayer windshield range between 0.040 inch (1mm) and 0.250 inch (6 mm) and said layer of flexible plastic interlayermaterial is of substantially uniform thickness between 0.015 inch (0.4mm) and 0.060 inch (1.5 mm).

The following test was performed to evaluate the relative impactresistance of laminates containing undistorted polyvinyl butyralinterlayers between two glass sheets such as result from the presentinvention compared with that of similar laminates containingdifferentially stressed polyvinyl butyral interlayers. A first set ofcontrol laminates 12 inches (30 cm) square containing 2 sheets of floatglass and a layer of plasticized polyvinyl butyral taken from acontinuous roll having a nominal thickness of 30 mils (0.76 mm) anddifferentially stretched in the manner of stretching a continuous rollof dyed polyvinyl butyral required to develop a curved cut-off linebetween dyed and undyed portions of a shaded windshield interlayer werefabricated. The fabrication included standard laminating technique forsample fabrication involving prepressing followed by a final laminationin an oil autoclave at 200 psi and 275° F. for 45 minutes to produce 25control specimens simulating specimens cut from the bottom portion of astandard production shaded windshield.

A set of 17 test specimens 12 inches (30 cm) square were also producedusing 2 plies of float glass and 30 mils (0.76 mm) nominal thicknesspolyvinyl butyral to simulate the clear portion of a shaded windshieldincorporating unstretched polyvinyl butyral interlayers using the samelaminating procedure as for the control specimens. All the glass sheetsin the specimens had the same nominal thickness.

The control specimens and the test specimens were subjected to impacttests to determine the mean break height from which a steel ball of 5pounds (2.27 kg.) mass would penetrate a specimen. The ANSI Code Z 26.1,1977, American National Standard Safety Code for Safety GlazingMaterials for Glazing Motor Vehicles Operating on Land Highways,requires that 10 out of 12 specimens hold a 5 pound steel ball from adrop height of 12 feet. From the results obtained in these tests, the 42specimens tested would be expected to pass the ANSI Code requirement.The results of the impact tests are as follows.

    ______________________________________                                        MEAN HEIGHT PENETRATION TEST RESULTS                                          Type of  Number of  Mean                                                      Specimen Specimens  Penetration Height                                                                           Sigma                                      ______________________________________                                        Control  25         18.18 feet     1.74                                       Test     17         16.66 feet     1.22                                       ______________________________________                                    

The results of the penetration test conclusively proves the superiorityof test specimens comprising unstretched interlayers to penetrationcompared to control specimens comprising differentially stretchedinterlayers. For laminated windshields having shade bands, the cleararea of the prior art windshields that require differential stretchingis less resistant to penetration on impact than the equivalent area ofshaded laminated windshields produced with interlayers processedaccording to the present invention.

The form of the invention shown and described in this disclosurerepresents an illustrative preferred embodiment thereof. It isunderstood that various changes may be made without departing from thegist of the invention as defined in the claimed subject matter whichfollows.

I claim:
 1. A method of fabricating a maximum number of individualsheets of substantially uniform thickness of trapezoidal shape of aflexible material of plastic composition suitable for use as aninterlayer for laminated windows having a graded coating along an upperportion thereof from a continuous ribbon of clear, flexible interlayermaterial of plastic composition having a substantially uniform thicknesscomprising:(1) cutting successive portions of said continuous ribbon ofclear, flexible interlayer material into a number of interfitting sheetsof trapezoidal shape while supporting each said successive portion ofsaid continuous ribbon in such a manner as to avoid wrinkling anddifferential stretching thereof during its cutting from said ribbon, (2)orienting each of said sheets of trapezoidal shape so formed into apredetermined orientation and positioning each said sheet in turn whileat said predetermined orientation in a predetermined position withrespect to apparatus to apply a coating composition along a preselectedlongitudinal portion only of said sheet of trapezoidal shape, (3)applying a coating of graded intensity along said longitudinal portiononly of said sheet of trapezoidal shape in such a manner that theintensity of said coating decreases transversely of the length of saidlongitudinal edge portion away from the longitudinal edge of saidsheet,and (4) continuing to support said sheet of trapezoidal shape insuch a manner as to avoid wrinkling and differential stretching thereofduring said coating applying step, whereby each said coated sheet oftrapezoidal shape with a coating of graded intensity so produced has asubstantially uniform thickness and is smooth and has a substantiallyuniform stress throughout its entire extent.
 2. A method as in claim 1,further comprising establishing a pair of stacks of alternate sheets oftrapezoidal shape formed from said continuous ribbon and orienting eachof the stacks into said predetermined orientation prior to exposing onesheet at a time at said orientation for said step of applying saidcoating.
 3. A method as in claim 1, wherein said coating is applied bymoving coating composition applying apparatus relative to said sheet oftrapezoidal shape when the sheet of trapezoidal shape occupies saidpredetermined position.
 4. A method as in claim 3, including masking aportion of said sheet of trapezoidal shape by interposing shield meansbetween said sheet and said coating composition applying apparatus inclosely spaced relation to said trapezoidal sheet when said coatingcomposition is applied.
 5. A method as in claim 4, including masking aportion of said sheet of trapezoidal shape comprising an area having acurved cut-off line by interposing shield means having a curved boundarybetween said sheet of trapezoidal shape and said coating compositionapplying apparatus in closely spaced relation to said sheet oftrapezoidal shape when said coating composition is applied.
 6. A methodas in claim 1, wherein said plastic material is selected from the groupconsisting of plasticized polyvinyl butyral, polyurethane, ionomers andsilicones.
 7. A method as in claim 1, further including laminating saidcoated sheet of trapezoidal shape to a curved sheet of relatively rigid,transparent glazing material selected from the group consisting ofglass, polycarbonates, acrylic resins, hard polyurethanes and polyestersby assembling said sheets together and subjecting said assembly to heatand pressure.
 8. A method as in claim 7, wherein said curved sheet ofrelatively rigid, transparent glazing material is composed of glass. 9.A method as in claim 8, wherein said curved glass sheet has a convexmajor surface and a concave major surface and said coated sheet oftrapezoidal shape is assembled in facing relation to said concave majorsurface.
 10. A method as in claim 1, further including assembling saidcoated sheet of trapezoidal shape between a pair of curved sheets ofrelatively rigid, transparent glazing material selected from the groupconsisting of glass, polycarbonates, acrylic resins, hard polyurethanesand polyesters to form an assembly, and laminating said assembly bysubjecting said assembly to heat and pressure.
 11. A method as in claim10, wherein at least one of said curved sheets is composed of glass. 12.A method as in claim 11, wherein said curved glass sheet has a majorconvex surface and a major concave surface and said coated sheet oftrapezoidal shape is assembled in facing relation to said major concavesurface.
 13. A method as in claim 10, wherein both of said curved sheetsare composed of glass.